In the production of cement clinker the exhaust gases from the final burning zone, which is generally formed by a rotary kiln, frequently contain a greater or lesser proportion of nitrogen oxides (NO.sub.x) which are formed partly by reaction of oxygen with the nitrogen in the combustion air and partly by oxidation of the nitrogen compounds in the fuel (cf. German periodical "Zement-Kalk-Gips" 1984, p. 499-507). This NO.sub.x content in the exhaust gases is undesirable because of the emission problems which it causes. Therefore various methods and plants have already been developed in order to reduce this NO.sub.x content in the exhaust gases.
It is known, for example, from "ICS proceedings" 1979, p. 45, FIG. 6, in the precalcination of cement raw material to produce a reducing atmosphere in the precalcination zone outside the rotary kiln by sub-stoichiometric combustion of fuel, and in this case the calcination of the preheated material is carried out in two stages: The first stage is operated with exhaust air from the cooling zone with an oxygen deficiency so that a CO-containing gas mixture from this precalcination zone enters the kiln exhaust gas duct in which a reducing zone is thus created in which a proportion of the NO.sub.x is reduced to nitrogen. In the adjoining part of the kiln exhaust gas duct combustion conditions are created by introducing a further proportion of exhaust air from the cooling zone so that the residual burn-out of the fuel takes place.
DE-OS-35 22 883 also describes a process in which additional fuel is introduced into the kiln exhaust gases outside the rotary kiln before the exhaust gases enter the precalcination zone, the quantity of this additional fuel and the oxygen content of the kiln exhaust gases being such that a sub-stoichiometric combustion in the exhaust gases from the burning zone takes place in the zone before the entry of the exhaust gases into the precalcination zone.
The earlier German Patent Application (P 40 00 795.2) also relates to a process which makes it possible by using very effective and at the same time economic means to produce a marked reduction in the NO.sub.x content of the exhaust gases and at the same time to purify the exhaust gases of troublesome heavy metals and trace pollutants. For this purpose the exhaust gases from the preheating zone are passed through a multi-stage filter zone, where the precipitation of dust contained in the exhaust gases takes place in a first filter stage, whilst in a succeeding further filter stage an adsorbent which is suitable for fixing NH.sub.x compounds, heavy metals, trace pollutants and/or SO.sub.2 is provided, and at least a proportion of this adsorbent is removed from the filter stage at specific intervals of time or at a specific charge with pollutants and introduced into the final burning and/or preheating zone. In this case the NH.sub.x compounds are present in the exhaust gases from a clinker burning plant either in the form of aerosols or of gas constituents). The adsorbent (for example carbonaceous material, zeolite or nitrogen-containing material) also serves for adsorption of sulphur dioxide (SO.sub.2). In the burning process for producing clinker the NH.sub.x compounds fixed by the adsorbent are again released and make a substantial contribution to the reduction of the NO.sub.x compounds produced in the clinker burning process. Thus in this way a marked reduction in NO.sub.x is already achieved during the clinker burning process and the denitration capacity to be provided in the filter zone is reduced.
The object of the invention is to make further improvements to the method and apparatus particularly with regard to the efficiency and economy of the exhaust gas purification using relatively simple means.